One of the greatest achievements in human space flight is also one that history overlooks. After the Mercury program—when the U.S. first launched astronauts into space—and before the Apollo program, when we went to the moon, there was the Gemini program, a 20-month, 10-mission sprint that saw NASA achieve things the U.S. had never even tried before. It was Gemini that taught us to walk in space, to rendezvous and dock in space, to fly long-duration missions that stretched 14 days in space—all essential skills before we could set off for the moon. There was triumph in Gemini, danger in Gemini, death and near-death in Gemini. The story of Gemini has never been fully told. Now, in the new book, Gemini, Stepping Stone to the Moon, the Untold Story, TIME’s Jeffrey Kluger, co-author of Apollo 13 and author of Apollo 8, changes that. The following excerpt has been edited for length.
The last thing Tom Stafford wanted to do was cut Gene Cernan loose in space. Stafford liked Cernan; he had trained hard with Cernan. For more than a year, the two of them had worked together to get ready for their three-day flight of Gemini 9, and now, in early June 1966, they were actually aloft. But the business of cutting Cernan loose was all at once a very real possibility.
Stafford, the commander of the mission, was inside the spacecraft, buckled into his left-hand seat. Cernan, the junior pilot, was outside, dangling—actually spinning, tumbling, and flailing—at the end of a long umbilical cord, completely unable to control his movements. It was Deke Slayton, the head of the astronaut office, who first raised the possibility of what Stafford should do in a situation like this. Shortly before launch, when Stafford and Cernan were in the suit-up room, preparing to climb into their pressure suits, Slayton appeared. Completely ignoring Cernan—not even making eye contact with the rookie astronaut—he addressed Stafford. “Tom,” he said, “I need to have a few words with you in private.” Cernan looked at Stafford with a questioning expression; Stafford merely shrugged in response, then followed Slayton out of the room. There, Slayton–speaking in quiet tones—laid down what was NASA’s life-and-death law.
Only once before, on the flight of Gemini 4 just a year earlier, had an American astronaut walked in space, and that had been merely a 20-minute float outside the cabin door. Cernan’s spacewalk would be much more ambitious, lasting hours, with the astronaut climbing all over the spacecraft to deposit and collect experiment packages, before making his way to the rear end of the ship where an Astronaut Maneuvering Unit—an Air Force-built jet pack, known as the AMU for short—was stowed. Cernan would be expected to climb into the backpack and fly free in space, connected to the ship only by a long, thin, nylon tether. The entire exercise posed enormous risks and Slayton was well aware of the mortal math involved in that.
Up to now, NASA had launched 12 crews of men into space and all 12 had come home safely. NASA wanted to keep those numbers as close to perfect as possible. Sending two men into space aboard Gemini 9 and bringing two men home was the objective, of course. But if something happened to Cernan when he was free-floating outside—if he became incapacitated, unconscious or was otherwise beyond rescue—Slayton would not stand for Stafford playing the hero, remaining in space with the cabin door open, and dying along with his crewmate. In such a situation, Stafford was to disconnect the umbilical that linked his junior astronaut to the spacecraft, seal the hatch, and come home alone, leaving Cernan, a 32-year-old Naval aviator, to become nothing more than a lifeless satellite of the Earth.
Cut him loose, Slayton said to Stafford. If it comes to that, cut him loose.
Stafford nodded his understanding, left Slayton, and returned to the suit-up room.
“What was that all about?” Cernan asked.
“Everything’s fine, Geno,” Stafford answered. “No big deal.”
But now, one week later, it was a very big deal indeed—with Cernan in very big trouble.
On Sunday June 5, 1966, at 5:30 a.m. Houston time, two days after launch, Cernan began his spacewalk, or, what NASA preferred to call in the agency’s arid argot, his extravehicular activity—or EVA. It would take a lot of preparation. The ground crew equipped him with an eleven-page checklist that covered everything from donning a chest pack which would provide him with oxygen, power, and communications; to unstowing the 25-foot umbilical cord—which the astronauts nicknamed the snake—that would keep him safely attached to the Gemini spacecraft; to pressurizing the modified EVA space suits both he and Stafford were wearing.
Gemini astronauts who were flying missions in which no EVA was taking place could afford to wear lighter suits, since the cabin itself was pressurized with air, surrounding them with an artificial atmosphere. But once the hatch was opened and Cernan exited, both men would be exposed to the hard vacuum of space, and that required more robust suits—seven layers thick. Even before pressurizing his suit, Cernan found the umbilical cord almost impossible to manage in the weightless environment of the spacecraft. The infernal thing floated and twisted and tangled itself, resisting all of Cernan’s efforts to keep it rolled and controlled.
“Canary,” Stafford radioed down to the Canary Island tracking station, “you can inform Houston we’ve got the big snake out of the black box.”
Once Cernan and Stafford inflated their suits, things became even more difficult, involving both the challenge of maneuvering the snake and the simple matter of moving at all. As the suits were inflated to a pressure of 3.5 pounds per square inch, they hardened and stiffened, making maneuvering in them almost impossible. It took all of an astronaut’s strength merely to bend an elbow or flex a knee. For Stafford, this would present little problem, as he would remain seated inside the spacecraft throughout the EVA. For Cernan, who was supposed to maneuver balletically around the Gemini 9 spacecraft, it would be a different matter entirely.
Stafford depressurized the cockpit, matching the vacuum inside to the vacuum outside so that the hatch would not blow open and fly free from interior air pressure when it was unlatched. Then Cernan reached up to the hatch’s handle, opened the little door, and the trace amount of air that remained inside the spacecraft breathed itself out and away. Cernan tentatively raised himself up, placed his feet on his seat and stood in the open hatch. He gaped at what he saw. The twin windows in the Gemini spacecraft measured only six inches by eight inches, affording the astronauts enough of a view to conduct some narrow photo reconnaissance of Earth and maneuver their spacecraft throughout their orbits. But that peephole field of vision was nothing compared to what Cernan now had. Gemini 9 was flying over Baja California, and Cernan could see the blue of the water against the green-brown spit of land and the rusty red surface of the desert southwest stretching in all directions.
“Hallelujah,” Cernan exclaimed. “Boy, is it beautiful out here, Tom.”
“It sure looks pretty,” Stafford said, taking in the minimal view his little window afforded him.
“I’ll grab my Hasselblad and take a picture of that,” Cernan said. Then he emerged fully from the spacecraft and prepared to make his way along its flank to its aft end, where the AMU was stowed and waiting for him. The journey along the Gemini, which measured only 18 feet and five inches from bow to stern, proved to be well-nigh impossible.
NASA and Cernan may have had their own ideas about how to maneuver at the end of a 25-foot umbilical cord, but Isaac Newton had his too, and those prevailed. Every physical action Cernan took produced an equal and opposite reaction in the umbilical cord; if Cernan moved out, the snake pulled him in; if Cernan moved left, the snake flung him right. The out-of-control motion radiated down to the spacecraft itself, which began yawing and pitching in response to the force. Such unwanted motion would have normally called for Stafford to fire his thrusters and stabilize the ship, but he dared not do that with Cernan outside, where the thruster exhaust could burn through his suit.
Instead, it was up to Cernan to stabilize himself. He grabbed for Velcro patches NASA had attached to the exterior of the spacecraft to help him gain his purchase, but the whipsawing of the umbilical proved more powerful than the hold the Velcro could provide. He also reached for handholds that had been installed on the exterior of the ship, but they were positioned multiple feet apart—the thinking being that Cernan would have an easy glide alongside the spacecraft and the handholds would be necessary only in an emergency. Instead, he continued flopping around at the end of the umbilical, utterly helpless to control his own motions.
“You’re kind of rocking the boat,” Stafford radioed to Cernan from within the jerking Gemini.
“The snake is all over me,” Cernan radioed back “It’s pretty much of a bear to get at these things because the handrails are so far back.”
Finally, through a combination of extreme exertion, Newtonian dynamics, and no small amount of sheer dumb luck, Cernan managed to swing in the direction of the spacecraft, slammed into its flank, and grabbed hold of one of the handrails. Now, at last, he got some additional help. Toward the back of the craft, NASA had attached a long cable running to the end of the ship that Cernan could grab onto, hand over hand. That too was exhausting work, as he could move only a few inches at a time before stopping and gathering in the snake to prevent it from yanking him away from his tenuous hold on the cable.
Cernan had been outside for more than an hour now, enough to move from the daytime side of Earth, where the temperature on him and the spacecraft was a blistering 250°F, to the nighttime side, where it was a frigid -250°F, and back to the sunlit side. His spacesuit was designed to keep the heat and cold within a survivable range, but all it took was a few degrees above or below that limit to cause him to feel a sweltering heat or a chilling cold. Sweat now began to pour down his face and sting his eyes—though he was helpless to wipe them since he was sealed inside his suit and helmet. Worse, his visor began to fog up from the dampness of the sweat, obscuring his vision. On the ground, at a console in Houston, flight surgeon Charles Berry read Cernan’s heart rate at 155 beats per minute, or about what it would be if he were running up 120 stairs each minute.
“How are you doing now, Gene?” Stafford asked.
“OK,” Cernan answered. “I’m going to slow down and take a rest.”
Cernan allowed himself to catch his breath and, he hoped, slow his heart, and then inch-by-inch, his visor running with condensed moisture, made his way semi-blindly to the back end of the spacecraft, where the AMU, which the Air Force engineers still expected him to don and fly, waited for him. But when he reached the aft of the ship, he encountered a nasty—and potentially deadly—surprise. That end of the spacecraft had been the part that was attached to the Titan II rocket that had blasted the crew into space; when the rocket separated just before the Gemini capsule reached orbit, it left a sawtooth, razor-sharp spear of metal behind, an obstacle Cernan would have to climb around without slicing open his suit and suffering an instant and fatal depressurization.
He reported the problem to Stafford and then, ever so carefully, negotiated that knife edge. When he had gotten past it, he tumbled gratefully into a recessed area at the back of the ship, where, with all of his exertion, he could fight his rigid suit and bend it into a position that would allow him to sit. He looked to his right where the AMU was stowed—and quietly groaned at what lay ahead.
Flying the AMU meant more than just donning the backpack, firing it up and taking off. Attached to the unit was a 35-item checklist, each step of which had to be completed, in sequence, for the thing to fly. The first chore was to switch on the lights attached to the unit so that he could read the checklist. He threw the proper switch and only one of the little lamps worked. Squinting through the dim illumination and his sweat-covered visor, he did his best to follow the checklist, but the work of strapping into the contraption was exhausting, and Cernan began to pant. On Berry’s screen in Houston, the astronaut’s heart rate now read 180 beats per minute.
Next, according to plan, Cernan disconnected from the umbilical that attached him to the ship, and clipped on instead to one that was connected to the AMU. Immediately, to the flight surgeon’s alarm, the signal from the astronaut to the ground flickered out. Cernan’s heart could accelerate to the level of cardiac arrest and the Houston doctor would never know it. And his heart was accelerating indeed as the unfiltered sun poured over him and the recessed metal skillet that was the rear of the spacecraft.
“We’re really cooking back here,” Cernan gasped.
From Stafford’s window, he could see that Gemini 9 was approaching another sunset. “OK, Gene,” he said. “Nighttime coming your way shortly.”
But nighttime, Cernan suspected, would only present another problem, and he was right. No sooner did the spacecraft move into the shadowed part of the Earth and the temperature drop to -250°F than the sweat that covered his visor froze over, blinding him completely. Cernan leaned forward, rubbed his nose against the inside of the visor and opened a tiny hole in the ice. He could see the lights of Australia beneath him.
“How are you doing, Geno?” Stafford asked.
“Really fogged up here,” Cernan said, continuing to work as well as he could through the AMU checklist. The same poor connection to the AMU that was preventing data from Cernan’s biomedical sensors from reaching the ground also now disrupted the communications between the two astronauts.
“Can you see anything Geno?” Stafford asked. “Can you understand me? Geno? Geno? Yes or no?”
Cernan responded, but whatever he was saying was unintelligible. Stafford contemplated his and his pilot’s options. Another daytime was soon approaching, which would cook Cernan again, followed by another nighttime, which would freeze his visor solid once more. Cernan could not maneuver with the main umbilical, much less, Stafford guessed, with the untested AMU, and every additional minute he remained outside was another minute of mortal danger.
That, for Stafford, was it. He knew Cernan, and, after training with him for more than a year, understood the man’s mettle. Cernan would keep working back there, with his vision gone, his heartbeat trip-hammering, a razor-like piece of metal threatening to tear his suit open wide, the light and shadow of the day and night tormenting him, all the while trying to fly the Air Force’s cursed AMU if it killed him—which it might.
As commander of the ship, Stafford had the authority to make any decision that concerned the conduct of his mission and the welfare of his crew, even if the flight controllers on the ground didn’t agree. The EVA, he decided, was over.
“OK,” he said, partly to Cernan and partly to the ground. “No-go. The link is terrible. Did you understand? Geno? Do you hear me? I said no-go. We’re aborting.”
Cernan did hear him. He released a long breath—both with relief and with trepidation. Aborting the EVA was easy enough. Making his way blindly back around the jagged metal shard, moving along the side of the Gemini—finding the handholds and Velcro and the ship’s aft cable without the benefit of vision, all the while battling to keep his heart rate under control—was no small thing. Then too, there was the matter of folding himself back inside the tiny seat of his little spacecraft and getting the hatch closed while wearing a spacesuit that kept him as rigid as a mannequin. Gene Cernan had left Gemini 9 to walk in space. If his suit tore or he became incapacitated or he could not reenter the ship at all, there was no guarantee that that walk would ever end.
“I don’t think I’ll make it that way,” Cernan said, flicking his unseeing eyes back around the rear end of the ship toward the front. But that way, as both astronauts knew, was the only way. The comment was all Cernan said that sounded like surrender—but it was enough.
Stafford heard the transmission clearly and nodded silently and somberly. Inside his head, Slayton’s words echoed hauntingly. Cut him loose.
From GEMINI: Stepping Stone to the Moon, the Untold Story by Jeffrey Kluger. Copyright © 2025 by the author and reprinted by permission of St. Martin’s Publishing Group.
